1. Field of the Invention
The present invention relates to helical antennas, and, more particularly, to multifilar, circularly polarized helical antennas having an adjustable beam angle.
2. Description of the Related Art
In a typical satellite communication system, outgoing RF signals transmitted from a mobile terminal unit are received directly by the satellite. The satellite in turn retransmits the RF signals to a ground station that is connected by wire to a public switched telephone network (PSTN), which in turn routes the outgoing signals to either a conventional telephone or to another mobile terminal unit of a satellite or cellular network. Incoming signals from a conventional wired telephone are conducted from the PSTN to the satellite ground station, which in turn transmits RF signals to the satellite for retransmission to the mobile terminal unit. Thus, communication can be between two mobile terminal units or between a satellite mobile terminal unit and a conventional telephone connected to a PSTN, or between a unit, for example. In each of the aforesaid conditions, the communication is routed through a PSTN. Also, communication can be with a radio base station type ground station which communicates via terrestial RF with mobile radios, such as taxicabs.
Presently, satellite systems that cover large geographical areas typically use several satellites that follow different paths at low or medium altitudes so that at least one satellite is at all times covering the desired geographical area. From the stand point of receiving signals, the low and medium altitude satellites have the advantage of being able to transmit a signal that reaches a mobile terminal unit at the earth's surface with a relatively large amplitude and without appreciable fading. However, such satellite networks are limited in their coverage area per satellite.
It has been proposed, to provide a satellite communications network that utilizes a high altitude geosynchronous satellite which is capable of covering an area corresponding to a substantial portion of the North American continent, so that a total of approximately 6 satellite beams will cover the entire continent from Alaska to Mexico. The satellite for such a network will be approximately 22,600 miles above the equator and will be designed to operate in the L-Band of RF frequencies. For example, the frequency of the signal being transmitted to the satellite will be between 1626 MHz to 1660 MHz; and the frequency of the signal received from the satellite will be between 1525 MHz to 1559 MHz. Energy travelling this great distance undergoes huge attenuation such that the power flux density incident at the antenna of the mobile unit is approximately 10.sup.-14 watts per square meter.
An acceptable antenna for mobile units, such as automobiles, trucks and the like, must not only be capable of receiving and transmitting signals of the aforementioned character, but in addition, must meet the space and dimensional constraints, resistance to shock, and mechanical vibrations attendant to the operation of such mobile units. In addition, overland travel in relation to the geosynchronous satellite mandates angular beam adjustment ranging from near the horizon to nearly vertical.
Small diameter, circularly polarized, helical mast antennas represent a viable candidate for mobile units in such geosynchronous satellite communication systems from the standpoints of size and other physical requirements of the mobile units. Also it is known that such antennas make a conical beam in space in which the elevation angle is a function of and thus varies with the pitch (turns per unit length) of the conductive helical filars of such antennas. Although the pitch of a helix may be adjusted by twisting opposite ends relative to each other, the required flexibility of the conductive filars in a helical antenna, particularly such an antenna of an overall length to require helices of multiple turns, makes it difficult to maintain spacing between turns of a single helix and, more acutely, the spacing between turns of multiple helical filars. Moreover, in such antennas, very small changes in conductive filar or wire position can change the impedance along the length of the antenna by several wavelengths. Such a change of impedance along the length of the antenna can steer the antenna beam and cause other poor performance such as high backlobes, poor mismatch and the like.